1. Field of the Invention
The present invention relates to a machine tool with a cutting fluid filtration device, and more particularly, to a machine tool provided with a cutting fluid filtration device which has function of cleaning a filter used to remove chips and sludge mixed in a cutting fluid and capable of accurately detecting clogging of the filter at low cost.
2. Description of the Related Art
A cutting fluid filtration device used in a machine tool is provided with a filter configured to remove impurities, such as chips and sludge, which are produced during machining and mixed in a cutting fluid, thereby adversely affecting the performance and reliability of the machine tool. This filter is cleaned in order to maintain its decontamination performance and extend its life. In cleaning the filter, the cutting fluid sometimes may be passed in the direction opposite to its flowing direction for normal use so that backwashing can be performed to remove chips and sludge adhering to the filter. Cutting fluid filtration devices having the function of performing filter cleaning based on such backwashing are disclosed in Japanese Patent Applications Laid-Open Nos. 2006-272474, 2006-255833 and 2001-252847.
The following is a description of an outline of the backwashing operation disclosed in the above-described patent documents.
First, the backwashing filter and inflow- and outflow-side ducts of a filter container are closed so that the cutting fluid remaining in the filter and the filter container can not flow out. Then, compressed air is introduced into the filter container in the direction opposite to the direction of normal flow of the cutting fluid, and a discharge valve in a discharge duct in the filter container is opened. Thereupon, the cutting fluid pressurized by the compressed air flows in the opened discharge duct after passing through the filter. Impurities, such as chips and sludge, adhering to the filter are removed by means of this flow of the cutting fluid.
A prior art example of the cutting fluid filtration device having the filter cleaning function to remove chips and sludge adhering to the filter by backwashing (disclosed in the above-described patent documents) will be described with reference to FIG. 13.
A cutting fluid filtration device 1 is controlled by a controller (not shown). The controller actuates the filtration device 1, and drives a filter pump 11 to draw up a cutting fluid 19 containing chips through a duct 20 and introduce the cutting fluid filtered by a filter into a machine tool body 70 through a filter container 13. The filter (not shown) for removing the chips from the cutting fluid is attached to the inside of the filter container 13.
As the cutting fluid 19 containing chips and stored in a tank 10 continues to be decontaminated, the amount of chips adhering to the filter in the filter container 13 gradually increases. A differential pressure between inflow- and outflow-side ducts 21 and 22 of the filter is detected by a differential-pressure switch 14, and backwashing is started when a predetermined value is exceeded by the detected differential pressure.
In the backwashing operation, the drive of the filter pump 11 is first stopped so that the drawing of the cutting fluid 19 containing chips from the tank 10 is interrupted. In this state, the inflow- and outflow-side ducts 21 and 22 of the filter container 13 are closed by an inflow valve 12 and an outflow valve 18, respectively. Thereupon, the cutting fluid remaining in the filter container 13 cannot flow out into the tank 10 and the machine tool body 70.
Subsequently, an air supply valve 17 connected to an air source 16 is opened, and compressed air is introduced into the filter container 13 in the direction opposite to the direction of normal flow of the cutting fluid. Then, the air supply valve 17 connected to the air source 16 is closed, and a discharge valve 15 connected to the filter container 13 is opened. In this state, the cutting fluid pressurized by the compressed air passes through the filter in the filter container 13 in the direction opposite to the direction of normal flow of the cutting fluid, and then returns to the tank 10 through the discharge valve 15 in a duct 23. Chips and sludge adhering to the filter are removed by means of this flow of the cutting fluid. Further, the differential-pressure switch 14 may be replaced with a pressure sensor or flow sensor.
In the backwashing operation of FIG. 13 disclosed in the foregoing patent documents, whether or not to perform the backwashing is determined by a measured value obtained by the differential-pressure switch 14 (or pressure sensor). Since the switch or sensor of this type is expensive, however, this operation leaves a room for improvement in cost. This also applies to the case where a flow sensor is used.
Japanese Patent Application Laid-Open No. 2000-61774 discloses a technique in which filter clogging is detected by detecting the change of electric current of a pump for supplying a cutting fluid to a filter. The electric current change of the pump used in this detection technique is such that the electric current value also changes depending on the variation of the viscosity of the cutting fluid and the degree of degradation of the pump. It is difficult, therefore, to accurately detect the filter clogging with a change of the electric current value attributable to clogging distinguished from that caused by some other factor than clogging.
Further, Japanese Patent Application Laid-Open No. 2004-66425 discloses a technique in which the time of cutting fluid supply to a clean fluid tank is measured so that an abnormality can be identified and the timing of filter replacement and the like can be informed when a predetermined preset time is exceeded by the time required for the supply. According to this technique, an abnormality is identified by the cutting fluid supply time. If the detection is performed during machining operation of the machine tool using a large amount of cutting fluid, however, accurate abnormality identification sometimes cannot be achieved due to reduction of the cutting fluid in the clean fluid tank used during the machining.